The separation of many classes of compounds by selective adsorption is well known. Also, separations of various mixtures containing olefins into their components are known, e.g., the preferential adsorption of linear olefins over branched olefins using adsorbents such as ZSM-5 and silicalite from Dessau 4,309,281 and Grose et al 4,061,724, the separation of normal C.sub.4 olefins from isobutylene with silicalite and pentene-1 as desorbent from Neuzil et al U.S. Pat. No. 4,455,445, and the separation of normal C.sub.6 olefins from branched-chain and/or cyclic olefin hydrocarbons with silicalite and pentene-1 or butene-1 as desorbent from Kulprathipanja et al U.S. Pat. No. 4,486,618.
European Patent Applications 0 372 938 and 0 372 939 disclose a method for treating silicalite or ZSM-5 zeolites to enable the treated adsorbents to selectively adsorb n-olefins and n-paraffins from C.sub.9 -C.sub.19 hydrocarbon mixtures thereof with branched olefins, branched paraffins, aromatic hydrocarbons and sulfur-containing compounds without catalyzing reactions of the olefinic feed materials. The adsorbent is first treated with an acid and subsequently treated with a base to remove residual acidity, e.g., from the adsorbent itself or from the binder, such as silica, so as to reduce acid catalytic activity.
Neuzil et al U.S. Pat. No. 4,455,445 discloses the separation of normal C.sub.4 hydrocarbons from isobutylene with silicalite adsorbent. The adsorbed normal C.sub.4 hydrocarbons are desorbed with pentene-1. The patentees also suggest advantages of diluting the desorbent with a material which will not be selectively retained by the molecular sieve (i.e., not capable of acting as a desorbent), e.g., iso-octane.
Kulprathipanja et al U.S. Pat. No. 4,486,618 discloses the separation of normal C.sub.6 olefins from C.sub.6 branched-chain olefins and C.sub.6 cyclic hydrocarbons with crystalline silica molecular sieves and recovering normal C.sub.6 olefins by desorption with pentene-1 or butene-1. Iso-octane may be mixed with the desorbent to function as a carrier and diluent.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton et al U.S. Pat. No. 2,985,589 and a paper entitled "Continuous Adsorptive Processing--A New Separation Technique," by D. B. Broughton presented at the 34th Annual Meeting of the Society of Chemical Engineers at Tokyo, Japan on Apr. 2, 1969, both of which are incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system or by rotary disc valves, which are also known, e.g., shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated herein.
We have discovered that certain desorbents are superior to the n-olefin and n-paraffin desorbents of the prior art in the separation of normal and branched-chain olefinic products and the recovery of the product streams from the adsorbent, silicalite, used to separate the products. With this combination of adsorbent and desorbent, normal olefins are selectively adsorbed relative to branched-chain olefins and cyclic olefins in the feed and the adsorbed normal olefins can be desorbed with less "tailing" than prior art desorbents, e.g., olefins, paraffins, cycloparaffins. This desorbed extract stream is a valuable product which can be used for the production of alcohols. The relatively non-adsorbed branched-chain olefins and cyclic olefins are eluted as raffinate, and are also useful products, e.g., for making synthetic elastomers and as blending agents for gasoline.